Othello's trusted ancient. Truly evil, he abuses the trust, loyalty and love he gains from others in his burning desire to destroy Othello, his motivation drawn from resentment (over Cassio's promotion) and the entirely unsubstantiated belief that his general has committed adultery with his wife, Emilia. His speeches, delivered in the prose of the common man, and full of fitting animal imagery, show him as a corrupt and Machiavellian manipulator in a play peopled with idealistic and noble characters. He is tortured at the end, but refuses ever to speak again.
Summary Article: Iago
From Chambers Dictionary of Literary Characters